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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _func-poll:
+
+***********
+V4L2 poll()
+***********
+
+Name
+====
+
+v4l2-poll - Wait for some event on a file descriptor
+
+
+Synopsis
+========
+
+.. code-block:: c
+
+    #include <sys/poll.h>
+
+
+.. cpp:function:: int poll( struct pollfd *ufds, unsigned int nfds, int timeout )
+
+
+Arguments
+=========
+
+
+
+Description
+===========
+
+With the :ref:`poll() <func-poll>` function applications can suspend execution
+until the driver has captured data or is ready to accept data for
+output.
+
+When streaming I/O has been negotiated this function waits until a
+buffer has been filled by the capture device and can be dequeued with
+the :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. For output devices this
+function waits until the device is ready to accept a new buffer to be
+queued up with the :ref:`VIDIOC_QBUF` ioctl for
+display. When buffers are already in the outgoing queue of the driver
+(capture) or the incoming queue isn't full (display) the function
+returns immediately.
+
+On success :ref:`poll() <func-poll>` returns the number of file descriptors
+that have been selected (that is, file descriptors for which the
+``revents`` field of the respective :c:func:`struct pollfd` structure
+is non-zero). Capture devices set the ``POLLIN`` and ``POLLRDNORM``
+flags in the ``revents`` field, output devices the ``POLLOUT`` and
+``POLLWRNORM`` flags. When the function timed out it returns a value of
+zero, on failure it returns -1 and the ``errno`` variable is set
+appropriately. When the application did not call
+:ref:`VIDIOC_STREAMON` the :ref:`poll() <func-poll>`
+function succeeds, but sets the ``POLLERR`` flag in the ``revents``
+field. When the application has called
+:ref:`VIDIOC_STREAMON` for a capture device but
+hasn't yet called :ref:`VIDIOC_QBUF`, the
+:ref:`poll() <func-poll>` function succeeds and sets the ``POLLERR`` flag in
+the ``revents`` field. For output devices this same situation will cause
+:ref:`poll() <func-poll>` to succeed as well, but it sets the ``POLLOUT`` and
+``POLLWRNORM`` flags in the ``revents`` field.
+
+If an event occurred (see :ref:`VIDIOC_DQEVENT`)
+then ``POLLPRI`` will be set in the ``revents`` field and
+:ref:`poll() <func-poll>` will return.
+
+When use of the :ref:`read() <func-read>` function has been negotiated and the
+driver does not capture yet, the :ref:`poll() <func-poll>` function starts
+capturing. When that fails it returns a ``POLLERR`` as above. Otherwise
+it waits until data has been captured and can be read. When the driver
+captures continuously (as opposed to, for example, still images) the
+function may return immediately.
+
+When use of the :ref:`write() <func-write>` function has been negotiated and the
+driver does not stream yet, the :ref:`poll() <func-poll>` function starts
+streaming. When that fails it returns a ``POLLERR`` as above. Otherwise
+it waits until the driver is ready for a non-blocking
+:ref:`write() <func-write>` call.
+
+If the caller is only interested in events (just ``POLLPRI`` is set in
+the ``events`` field), then :ref:`poll() <func-poll>` will *not* start
+streaming if the driver does not stream yet. This makes it possible to
+just poll for events and not for buffers.
+
+All drivers implementing the :ref:`read() <func-read>` or :ref:`write() <func-write>`
+function or streaming I/O must also support the :ref:`poll() <func-poll>`
+function.
+
+For more details see the :ref:`poll() <func-poll>` manual page.
+
+
+Return Value
+============
+
+On success, :ref:`poll() <func-poll>` returns the number structures which have
+non-zero ``revents`` fields, or zero if the call timed out. On error -1
+is returned, and the ``errno`` variable is set appropriately:
+
+EBADF
+    One or more of the ``ufds`` members specify an invalid file
+    descriptor.
+
+EBUSY
+    The driver does not support multiple read or write streams and the
+    device is already in use.
+
+EFAULT
+    ``ufds`` references an inaccessible memory area.
+
+EINTR
+    The call was interrupted by a signal.
+
+EINVAL
+    The ``nfds`` argument is greater than ``OPEN_MAX``.